1. Field of the Invention
The present invention is directed to reinforced polysiloxane elastomer compositions of improved mechanical properties. More specifically the present invention is directed to reinforced polysiloxane elastomer compositions which have improved creep resistance, improved compression set and improved crush resistance and are eminently suitable for use as insulators for leads of implantable medical devices, particularly cardiac pacemakers.
2. Brief Description of the Prior Art
Polysiloxane elastomer compositions have been known in the art for a long time. Many polysiloxane elastomer compositions of the prior art contain a silica reinforcer that has been treated to make it compatible with the generally hydrophobic polysiloxane matrix of the elastomer. Prior art pertaining to polysiloxane materials can be found in U.S. Pat. Nos. 3,341,490, 3,284,406, 3,457,214, 3,996,187, 3,996,189, 4,615,702, 5,236,970 and in European Patent application No. 0110537 filed on Oct. 18, 1983.
Silicone elastomer rubber tubing that contains silica reinforcing material has been extensively used as insulator on electrical leads, particularly on leads of implantable medical devices, perhaps most importantly from the standpoint of the present invention, on leads of cardiac pacemakers. An early version of the reinforced elastomer used as insulators on implantable devices, primarily pacemakers, was known in the trade as Dow Coming MDX4-4516. A later version was known as Dow Coming HP (high performance) material. The early version (MDX4-4516) consisted of silica reinforced polydimethylsiloxane which included approximately 0.142 mol per cent methylvinylsiloxane [(CH.sub.3)(CH.sub.2 .dbd.CH)Si--O] units. The end blocking (terminal) units of this early version of the polymer were dimethylvinylsiloxane units [(CH.sub.3).sub.2 (CH.sub.2 .dbd.CH)Si--O]. Based on the presence of approximately 0.142 mol per cent of methylvinylsiloxane units in this prior art polymer, it can be calculated that approximately 750 dimethylsiloxane [(CH.sub.3).sub.2 SiO] units are disposed on the average between each "pendant" vinyl group in the polysiloxane chain. The vinyl groups in the polymer participate in a cross-linking reaction which occurs in the final curing step when the polymer is formed into a desired shape, such as a single or multi-lumen tubular object. The curing or cross-linking step was initiated by addition of a peroxide catalyst or catalysts. The filler material of this earlier version was silica that had been treated with dimethylsiloxane oligomer.
The later, high performance version of the prior art silicone elastomer (Dow Corning HP) used as leads in implantable medical devices, primarily pacemakers, consisted of a blend of a first (major) and a second (minor) polysiloxane composition. The first and major composition comprised approximately 80-90 per cent (by weight) of the blend, and this composition contained no pendant vinyl or other pendant olefinic groups. The second and minor composition comprised the balance of the blend (before reinforcing treated silica was added), and had approximately 2 mol percent methylvinylsiloxane [(CH.sub.3)(CH.sub.2 .dbd.CH)Si--O] units and was also terminated with dimethylvinylsiloxane [(CH.sub.3).sub.2 (CH.sub.2 .dbd.CH)Si--O] units. The reinforcing material was silica that had been treated with a reagent that introduced trimethylsilyl groups into the material, thereby replacing OH functions with OSi(CH.sub.3).sub.3 functions and rendering the treated silica compatible with the "hydrophobic" silicone polymer. The curing or cross-linking step was initiated by addition of a platinum catalyst or catalysts.
As it will be readily appreciated by those skilled in the art, certain mechanical properties, such as tear strength, abrasion resistance, resistance to shredding, compression set, crush and creep resistance are of great importance in the materials for electrical leads in any device that is implanted into the human body, and particularly so for insulators of cardiac pacemakers. It should also be readily appreciated by those skilled in the art that improved mechanical properties, and particularly improved compression set, creep and crush resistance allow the manufacture of insulated electrical leads of smaller dimensions and therefore facilitates "downsizing" of the implantable device. Whereas the later version (Dow Corning HP) of the above-summarized prior art silicone elastomers had certain improved mechanical properties (for example improved tear strength) relative to the earlier MDX4-4516 version, it was then surprising to the artisans in the field that this otherwise improved material had less crush resistance than the earlier MDX4-4516 material. Therefore, up to the present invention the prior art struggled with the problem that neither of the two types of silicone elastomeric materials available for forming insulators for cardiac pacemaker and similar implantable leads had optimal characteristics. As noted above, the earlier version could have used improvement in several mechanical properties, and the later version had improved mechanical properties in virtually all aspects, but had less crush resistance than the earlier version.
In light of the foregoing, up to the present invention the need still existed in the prior art for a polysiloxane elastomer material which is suitable for use as insulator for leads of implantable electrical devices, particularly cardiac pacemakers, and which has improved overall mechanical properties, including improved compression set, and improved crush and creep resistance. The present invention provides such a material.